Place a 10-inch cast iron skillet on your camp stove over medium-low heat. Add the drained, cut peaches, 2 tablespoons brown sugar, cinnamon, and nutmeg. Stir gently and let cook for 3 minutes until the peaches soften and the sugar dissolves.
In a small bowl, mix the rolled oats, flour, and remaining 1/4 cup brown sugar. Add the cold butter cubes. Use your fingertips or a fork to work the butter into the dry mixture until it resembles coarse crumbs. Stir in the nuts if using.
Spread the crumble topping evenly over the hot peaches in the skillet. Do not stir. Cover the skillet with a lid or a piece of heavy-duty aluminum foil pressed down around the edges.
Cook over low heat for 10 minutes. Lift the lid carefully — steam will escape. The topping should look set and lightly golden. If it's still pale, cook uncovered for 2 more minutes.
Remove the skillet from the heat. Let it rest for 2 minutes. The filling will thicken as it cools slightly. Serve warm, straight from the skillet.